Technical Field
Embodiments of the invention relate generally to magnetic resonance imaging (MRI). Particular embodiments relate to methods and apparatus for limiting (“decoupling”) unwanted electromagnetic interaction or coupling of a receive coil with a transmit coil.
Discussion of Art
Generally, magnetic resonance images are obtained by imposing on a target object, such as a patient's body, a large uniform magnetic field (“B0”) from a “field” or “polarizing” coil. This large uniform field substantially aligns the quantum spins of protons in the molecules within the target object, although the spins of protons within chemically distinct molecules will continue to precess at distinct Larmor frequencies. By briefly imposing a pulsed RF field (“B1”) from a “transmit coil,” generally transverse to B0, it is possible to excite the protons of molecules with spins that precess at a Larmor frequency matching the pulsed RF. As the excited protons relax back to their lower energy normal state, they emit RF energy that can be detected by a “receive coil,” which may the same as, or separate from, the transmit coil. The detected RF energy is recorded as intensity data that then is processed, by known means, so as to obtain a visual approximation or image of where and how the various chemicals are disposed within the target object.
Often, the transmit and receive coils are a single mechanical and electrical structure or array of structures, with transmit/receive mode switchable by auxiliary circuitry. For enhanced image quality, however, it may be desirable to provide a receive coil that is mechanically and electrically isolated from the transmit coil. In such case it is highly desirable to provide that the receive coil, in its receive mode, will be electromagnetically coupled to and resonant with the RF pulse that was provided from the transmit coil. Further, it may be desirable to provide that the receive coil is electromagnetically decoupled from and therefore not resonant with the transmit coil, during actual transmission of the RF pulse. Such decoupling averts a potential problem of noise produced within the auxiliary circuitry when the receive coil couples to the full power of the RF pulse.
In view of the above, it is deemed desirable to provide apparatus and methods for coupling and decoupling a receive coil rapidly and with relatively low circuit losses. Such apparatus and methods might also be helpful toward operation of RF transceivers, generally.